The invention relates to a platen heated by electromagnetic induction. The invention is adapted for use with a press in order to join and/or shape materials by the application of heat and pressure.
Heating platens are used in hot platen presses to heat tooling, sheet metal parts, parts to be diffusion bonded, parts to be superplastically formed and many other parts which require the application of pressure in order to join and/or shape the parts. It is desirable that such platens transfer heat uniformly to the workpiece and be capable of sustaining high compressive loads.
Generally, most prior art heating platen systems incorporate electrical resistance methods of heating. For example, U.S. Pat. No. 3,393,292 to Ritscher discloses a metallic platen using electrical resistance heating rods. The heating rods are positioned in recesses in a pressure plate and are unevenly spaced to compensate for heating losses at the edge of the plate. The primary shortcoming of this system, notwithstanding the positioning of the heating elements, is its inability to provide the required watt density to meet the requirements of efficient, cost effective, high temperature metal working.
U.S. Pat. No. 3,528,276 to Schmidt, et al, uses a cored metal platen to uniformly distribute and control the heat. Electrical resistance type heating elements are used. In addition, a liquid metal fills the bores in the platen in order to enhance heat transfer to the platen. Since this apparatus has a 1500.degree. F. upper temperature limit, it cannot adequately support high temperature metal working operations. Thus, as exemplified by the Ritscher and Schmidt devices, the use of conventional resistance heaters as a primary heat source is inadequate for high temperature metal working operations.
Other prior art systems incorporate electrical resistance heating elements which are embedded in a ceramic platen. For example, U.S. Pat. No. 3,754,499 to Heisman, et al., discloses silicon carbide heating rods encased in ceramic which functions as a heat sink. Although the ceramic is used as a heat and electrical conductor, it is basically an insulator; thus, the ceramic is not able to transfer heat to the workpiece as well as metal heat sinks which are directly coupled to heating rods. Consequently, a primary disadvantage of this system is that it is not able to support rate production in the higher temperature ranges. Moreover, due to its inefficient method of heating and high maintenance requirements, it is limited to incorporation with relatively small platens. In addition, due to the slow and inefficient heating of the platen, the length of time at which the platen is required to be at or near the desired temperature tends to shorten the life of the heating platen system and increase the likelihood of atmospheric contamination.
Other prior art systems have used intermediary materials between the heating elements and the platen in order to provide more uniform heating of the platen. Such a system is exemplified by U.S. Pat. No. 3,478,192 to Fink. Fink discloses plates which are heated by electrical resistance elements. Oil circulates through the plates to equalize the temperature throughout the plates. The main disadvantage with this prior art system is that the heated oil concept embodied therein will not practically perform above 500.degree. F. and therefore cannot support high temperature metal working operations.
A heating platen system is thus needed that will provide fast and efficient heating of tooling, sheet metal parts, parts to be diffusion bonded, parts to be superplastically formed and many press applications where materials are joined and shaped under heat and pressure.